It is the long range purpose of this work to clarify the mechanism of cell transformation by studying the genetic and biological factors affecting carcinogenesis by chemicals. This work also aims to clarify the role of these factors in the etiology of human cancer incidence. The topic of present interest is the dissociation of the susceptibility of variants of BALB 3T3 cells to the induction of the transformation by chemical carcinogens and ultraviolet light irradiation from their susceptibilities to the induction of mutation by these agents and to the viral transformation.